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Turntable Strobe dot wobbles

C

charlie.simm

New Member
I have a 1970's Technics SL-220 that is working probably good enough but the strobe dots move back and forth a very small amount. The wow measures +/-0.12 % RMS as measured with a RPM WOW phone app. I've replaced the belt twice and without significant change in the small movement. The movement is ~<1/32" "peak to peak".

My question is what should I look at, try, or purchase?
Am I asking too much from a 1/2 century old turntable. It was mine way back when.

I've suspected a cheap belt. Could this be the difference between a molded and cut belt?
I wonder if the platter pulley (belt rim on the underside of the platter) could be out of round some how.

More info on the symptom:
  1. At startup the variation is very significant
  2. As the turntable keeps running the variation gradually gets to the 1/32". This is really hard to measure.
  3. If I put a turntable weight on it. I think (or hope) the variation cuts down. This is hard to see at ~1/32"
  4. The RPM WOW app also shows the speed measurements on a graph. This graph shows a peak variation at the RPM rate. See the attached pic.
  5. The strobe dots appear fuzzy.
  6. I've replaced the neon strobe lamp.
  7. The turntable seems to spin flat. In other words, there doesn't seem to be much height variation at the rim. This is by eye.
  8. By eye, there doesn't seem to be much run out on the turntable.

Context:
  1. The turntable speed pots were de-oxit last fall.
  2. ALL electrolytic caps were replaced.
  3. All old grease and lube was cleaned and redone with a synthetic grease and lube.
  4. I've cleaned the motor and turntable pulleys.
  5. The turntable spins and spins and spins. This is without the belt on.
  6. Transported back and forth from college to home so maybe a little rough handling?

WOW Phone app: I'm very careful about placing the phone on the turntable for the WOW phone app. I use a masking tape roll and center it as best I can. I put the phone on the roll and center as best I can.

Me: EE background. Some EE tools including oscilloscope. I have a magnetic runout gauge. I don't think I can hear the wow as I don't have a trained ear.
1787443765190.jpeg
 
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I have heard of belts that were not of a uniform thickness causing issues as described. Out of curiosity were the two replacement belts from the same vendor? Have you looked at the drive spindle for contamination of some sort. Issue with the drive spindle face is probably not very likely, but if the belt is slipping part turn?
 
Willeve146 said:
I have heard of belts that were not of a uniform thickness causing issues as described. Out of curiosity were the two replacement belts from the same vendor? Have you looked at the drive spindle for contamination of some sort. Issue with the drive spindle face is probably not very likely, but if the belt is slipping part turn?
Click to expand...
Thanks Willeve146, One was from Amazon, one from LP Gear. Good Idea.
 
Good belts are hard to find, and they make a big difference on speed stability. 0.2 wow is just at the audible limit for music
 
Strobe dots can move back and forth as they are cast slightly wrong, this is totally normal.
Phone apps are not accurate at all as they are not calibrated. Add to this a phone puts the platter out of balance itself making wow and flutter bigger.
If the reading you are getting was accurate ( it's not) then that would be quite good for a belt drive.
Chris
 
charlie.simm said:
I have a 1970's Technics SL-220 that is working probably good enough but the strobe dots move back and forth a very small amount.
Click to expand...

Just understand the "strobe" light on your TT is just a neon on the primary side (mains). Mains frequency is not perfectly rock solid 50Hz or 60Hz. It varies quite significantly, up or down a Hz or so all the time. The average over an X hour period is the nominal frequency in your country. As such, you will never get a mains derived strobe being perfectly still on a TT, even if the FG and the drive system is "perfect".

1787609921814.png

If you put a high resolution frequency counter on your mains and plot the frequency vs time, you would see our mains is not anywhere near as "accurate" as we would hope. The power companies are constantly fighting to maintain frequency on the grid all over the world. They do this by controlling generation to match supply and demand. This is AEMO's (Australia). It works on a 5 minute cycle of prediction of demand and supply. Notice the normal range.

1787610916483.png

1787610492585.png

Your TT is a belt drive FG servo, so the rotation could be accurate and yet the "strobe" will wander a bit, back and forth or drift a touch. Don't obsess about it- your TT is most likely spot on.

The only "strobes" that sit perfectly still on TTs are derived from divided down quartz oscillators and PLLs (usually on direct drive TTs). They create their own reference strobe which is way more accurate than a simple neon hanging off the mains.
 
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Hi Charlie,
I use the same app and if you place the phone centered on the spindle (I use a Nagaoka 45 adapter that raises it above the spindle) the results are consistent, so it will give you a good idea as to any improvements. Just laying the phone on the platter will affect the wow significantly.
However, if the phone is placed correctly 0.12% WRMS is pretty bad, also for a belt drive, and I would expect you to be able to achieve 0.07-0.05% (or better, Technics specifies 0.045% WRMS) with a good belt (most of the aftermarket ones are not so great these days).
However there are several factors that may have an effect:
  • The motor and pulley can be ruled out as the variation frequency is much too low (as previously mentioned)
  • With this pattern I would not suspect the speed switch or pots as it repeats once per rotation
  • The spindle bearing may have wear. If you can hear the slightest scraping noise when rotating the spindle without the belt you may have scoring on the spindle. This can be polished away with Autosol (chrome polish) or similar. Even if it's not immediately audible there my still be a wear pattern that causes increased wow. It's easy to check by taking the bearing apart. There should be no scoring of the surface. I've seen this issue several times.
  • Incorrect lube for the spindle may also have an effect. It has to be thick enough as it's an open bearing
  • Your belt is of uneven thickness (I've had belts that varied by 0.15 mm, and somme that came with burrs from the cutting!). You can check with calipers.
  • Some belts come with a name printed on one side (usually in white). I've found that this may affect the elasticity/performance slightly, and always remove it.
I assume that you have cleaned the platter where the belt contacts it, but as previously mentioned its also a suspect.
 
The problem is that what the phone apps display most likely is not a proper weighted W&F DIN , nor Wrms. Many just display the the variation within of +/-95% of the samples deviation . The last is called W&F 2S and is pretty good parameter to compare. Just note that A 95% variation value of 0.2 can correspond to a wrms value of 0.07 or lower/higher due to the weighting filter used. A Japanese trick to make w&f values look low.

My Direct drive is shown below, my belt drive can vary between 0.05 and 0.15 over time , if it is above 0.1 it is time to clean the belt and pulleys and treat the belt with talcum.or adjustment of the position of belt or the separate motor pod. Levelling of the turntable /platter /motor pod can make a big difference too.

IMG_0319.jpeg
 
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Jens Sinding said:
Hi Charlie,
I use the same app and if you place the phone centered on the spindle (I use a Nagaoka 45 adapter that raises it above the spindle) the results are consistent, so it will give you a good idea as to any improvements. Just laying the phone on the platter will affect the wow significantly.
However, if the phone is placed correctly 0.12% WRMS is pretty bad, also for a belt drive, and I would expect you to be able to achieve 0.07-0.05% (or better, Technics specifies 0.045% WRMS) with a good belt (most of the aftermarket ones are not so great these days).
However there are several factors that may have an effect:
  • The motor and pulley can be ruled out as the variation frequency is much too low (as previously mentioned)
  • With this pattern I would not suspect the speed switch or pots as it repeats once per rotation
  • The spindle bearing may have wear. If you can hear the slightest scraping noise when rotating the spindle without the belt you may have scoring on the spindle. This can be polished away with Autosol (chrome polish) or similar. Even if it's not immediately audible there my still be a wear pattern that causes increased wow. It's easy to check by taking the bearing apart. There should be no scoring of the surface. I've seen this issue several times.
  • Incorrect lube for the spindle may also have an effect. It has to be thick enough as it's an open bearing
  • Your belt is of uneven thickness (I've had belts that varied by 0.15 mm, and somme that came with burrs from the cutting!). You can check with calipers.
  • Some belts come with a name printed on one side (usually in white). I've found that this may affect the elasticity/performance slightly, and always remove it.
I assume that you have cleaned the platter where the belt contacts it, but as previously mentioned its also a suspect.
Click to expand...
Thanks Jens, this is very good information.

I've recleaned the belt, the motor pulley, and the platter pulley surface. No diff, or maybe a little bit.
Without the belt the platter spins and spins and spins. I'll check the spindle surface especially around the bushing.
I did notice that there is an extremely small amount of play between the bushing and the spindle.
Also, the platter seems to wobble on the spindle a little even though there both tapered.
The spindle chrome looks very clean unworn.
This play and spindle wobble maybe manufacturing tolerances?

Charlie
 
charlie.simm said:
Thanks Jens, this is very good information.

I've recleaned the belt, the motor pulley, and the platter pulley surface. No diff, or maybe a little bit.
Without the belt the platter spins and spins and spins. I'll check the spindle surface especially around the bushing.
I did notice that there is an extremely small amount of play between the bushing and the spindle.
Also, the platter seems to wobble on the spindle a little even though there both tapered.
The spindle chrome looks very clean unworn.
This play and spindle wobble maybe manufacturing tolerances?

Charlie
Click to expand...
I'm going to take the turntable a part to examine the spindle better and re-clean it. I'll "feel" it for issues versus just looking at it.
 
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